The Maillard reaction was reported for the first time in 1912 by Maillard who was a French biochemist. It is a nonenzymatic glycosylation (glycation) reaction which takes place between a reducing sugar, such as glucose, and an amino acid or protein. The reaction participates in browning, production of fragrant components, generation of good taste, denaturization of protein, etc. during heating or storage of food. Accordingly, the Maillard reaction has been mostly investigated by food chemists. In 1968, glycosylated hemoglobin (HbA1c), which is a small component of hemoglobin, was identified in the living body and was found to increase in diabetic patients. With this discovery, public attention has been drawn to the significance of the Maillard reaction in the living body. Its relation to the occurrence of adult diseases such as complications from diabetes and arteriosclerosis, and the progress of aging have been subjects of public interest.
Brownlee, M., et al., Science, Vol. 232, pp. 1629-1632 (1986) reported in 1986 concerning the Maillard reaction in a living body. The Maillard reaction can be divided into an early stage and an advanced stage. In the early stage an amino group of a protein and an aldehyde group of a reducing sugar form a Schiff's base, and then a stable product by an Amadori arrangement is produced via 1,2-enaminol. In the advanced stage, the above product is subjected to a long term reaction which produces advanced glycation endproducts (AGE) of the Maillard reaction characterized by fluorescence, browning and molecular crosslinking. This glycosylated product in the advanced stage produced by a crosslinking glycation of proteins has a lower solubility and is sclerosed whereby it is hardly metabolized by protease. As a result, the glycosylated product induces deposition into tissues or sclerosis, or denaturation of proteins which is a cause of the onset of various diseases.
There are many kinds of proteins which are crosslinked by means of the Maillard reaction in vivo. Proteins having slow metabolic turnover (such as collagen, elastin, hemoglobin, erythrocyte membrane, myelin, tubulin, LDL, fibrin, serum albumin, lens protein and renal glomerular basement membrane) are particularly apt to be polymerized in a crosslinking manner. With regard to a crosslinking reaction of protein by the Maillard reaction, the above-mentioned proteins having slow metabolic turnover are affected even if the blood sugar level is normal whereby aging and deterioration of the proteins are induced.
It is believed that such aging and deterioration of protein by the crosslinking glycation are important causes of the onset of various diseases such as adult diseases where the frequency of the onset increases with aging. Accordingly, if production of crosslinked protein (a glycated product in the advanced stage) is retarded or is substantially suppressed by a drug, the drug may be used for treatment or prevention of diseases accompanied by aging or those induced by deposition into tissues or sclerosis or denaturation of crosslinked protein. Such a drug may also be used to lengthen an animal's life span.
It has been known that an intermediate (3-deoxyosone) which is quite highly reactive with an amino group of proteins, etc. is produced from the above-mentioned 1,2-enaminol which is an intermediate in the Maillard reaction. 3-Deoxyosone corresponds to a mono-dehydrated substance of a reducing sugar and the substance which is produced from glucose or from fructose is 3-deoxyglucosone (3-DG). The substance which is believed to be most important in the production of AGE of the Maillard reaction is said 3-DG. Although various dicarbonyls are produced in the Maillard reaction, 3-DG is produced in the highest amount. As compared with glucose or the like, 3-DG is highly reactive with amino compounds and has a very high activity for resulting in a crosslinking of protein. The above-mentioned AGE is an index for the production of glycated protein. As a result of studies by many researchers, various candidate substances have been identified already. Among them, AGE compounds such as derivatives of imidazolone, pyrropyridine and pyralin are believed to be derived from 3-DG and it is becoming clear that 3-DG greatly participates in the formation of crosslinking in protein in vivo.
As mentioned above, production of a crosslinked protein induces aging and deterioration of protein existing in a living body is a factor which participates in the onset and worsening of various diseases. Therefore, substances which inhibit the Maillard reaction in vivo have been investigated. For example, Japanese Examined Patent Publication (JP) Hei-06/067,827 B. Publication (JP) Hei-06/067,827 A discloses inhibitors for the Maillard reaction which suppress the advanced stage glycosylation of a target protein by reaction with the carbonyl group of the early stage glycosylated product such as 3-DG. Although aminoguanidine which is mentioned as an effective component thereof is one of the substances which have been most extensively studied in this field, said compound has not been practically used as a pharmaceutical agent yet. In addition, thiosemicarbazides, 1,3-diaminoguanidine, benzoylhydrazine, etc. (JP Sho-64/056,614 A); carbazine derivatives (JP Hei-02/167,264 A); benzopyran derivatives (JP Hei-03/204,874 A); benzothiazole derivatives, benzimidazole derivatives, etc. (JP Hei-06/305,964 A); hydantoin derivatives (JP 06/135,968 A); imidazolidine derivatives (JP Hei-07/133,264 A); thiazolidines of a pyrazole type (JP Hei-08/157,473 A); etc. have been disclosed as useful Maillard reaction inhibitors.
In each of the above-mentioned patent publications, inhibitory activity towards the Maillard reaction was measured by means of the amount of crosslinkingly polymerized protein or AGE (intensity of fluorescence) produced, which is a final product of the Maillard reaction, as an index. However, the present inventors used the amount of 3-DG produced which is a most important and highly active intermediate in the crosslinking of protein in vivo as an index of inhibiting action towards the Maillard reaction. 3-DG is a causative substance participating in the crosslinking of proteins in a living body. Accordingly, drugs which inhibit the production of 3-DG are highly useful as excellent inhibitors for the production of crosslinked proteins.
The present invention provides novel inhibitors for the production of 3-deoxyglucosone (3-DG) which is an intermediate in the Maillard reaction and which induces a crosslinking glycation of proteins participating in various diseases.